As the resolution for integrated circuit patterns increases, higher performance is being demanded of projection optical systems that are used to project an image from a reticle or mask onto a semiconductor wafer or substrate. In current projection optical systems, in order to meet such demands, the resolving power of the projection optical system can conceivably be improved by increasing the numerical aperture (NA) of the projection optical system. In addition, such optical systems are normally double telecentric to avoid magnification errors and have the capability to vary the numerical aperture to obtain the appropriate conditions for imaging the integrated circuit patterns.
Referring to FIG. 1A, one problem associated with current projection optical systems is the existence of a certain amount of field curvature of pupil at the pupil location. This occurs because a mechanical aperture stop is in a plane, and when the numerical aperture is changed, phenomena such as asymmetrical vignetting occurs. As seen in FIG. 1A, a projection optical system may be broken down into two parts, one part on a first object side of the aperture stop having a first Petzval sum (ptz(1)), and another part on a second object side of the aperture stop having a second Petzval sum (ptz(2)). Referring to FIG. 1B, when the NA is decreased, asymmetrical vignetting occurs, resulting in a degradation of the imaging performance of the projection optical system. Phenomena such as asymmetrical vignetting cause degradation in the imaging performance of the projection optical system as a whole. Due to the current resolution limits required in projection optical systems, these degradations could be ignored. However, as systems seek to improve resolution and utilize projection optical systems having a higher NA (e.g., NA&gt;0.63), field curvature of pupil at the aperture stop and other asymmetrical phenomena present a much more serious problem and cannot be ignored.
Therefore, what is needed are projection optical systems having high numerical apertures and minimum field curvature of pupil in the aperture stop plane.